The Nature of Photovoltaic Cells
A photovoltaic cell, often referred to as a solar cell, is a semiconductor junction device which converts light energy into electrical energy. A typical photovoltaic cell is a layered structure comprising four principal layers: (1) an absorber-generator (2) a collector-converter (3) a transparent electrical contact, and (4) an opaque electrical contact. When light is shone onto the absorber-generator, the device generates between the two contacts a voltage differential which increases as the intensity of the light increases.
The absorber-generator (commonly referred to as the "absorber") is a layer of semiconductor material which absorbs light photons and, as a consequence, generates minority carriers. Typically, the absorber captures photons and ejects electrons thus creating pairs of negatively charged carriers (electrons) and positively charged carriers ("holes"). If the absorber is a p-type semiconductor, the electrons are minority carriers; if it is n-type, the holes are minority carriers. As minority carriers are readily annihilated in the absorber by recombination with the plentiful majority carriers, they must be transported to a region wherein they are majority carriers before they can be utilized to power an electrical circuit.
The collector-converter (the "collector") is a layer of material in electrical contact with the absorber wherein the majority carriers are of the same conductivity type as the minority carriers generated in the absorber. This layer "collects" minority carriers from the absorber and "converts" them into majority carriers. If the collector is an oppositely doped region of the same semiconductor as the absorber, the photovoltaic device is a p-n junction or homojunction device. If the collector is comprised of a different semiconductor, the device is a heterojunction; and, if the collector is metal, the device is a Schottky junction.
The transparent contact is a conductive electrical contact which permits light to pass through to the absorber. It is typically either a continuous transparent sheet of conductive material or an open grid of opaque conductive material. If the transparent contact is on the same side of the photovoltaic device as the absorber, the device is referred to as being in the front wall configuration. If the transparent contact is on the opposite side, the device is said to be in the back wall configuration.